The three-dimensional structure of the lysozyme from bacteriophage T4 has been determined by X-ray crystallography from a 2.4 A resolution electron density map. Also an extensive library of mutant enzymes is available, and techniques for obtaining additional mutants have been established. A number of mutant lysozymes have been crystallized isomorphously with the native protein. We propose to compare in detail by X-ray crystallography, nuclear magnetic resonance, and various optical methods the three-dimensional structures, the stability, the folding, and the activity of selected mutant enzymes with that of the wild type enzyme. In so doing we expect to obtain detailed information, not previously accessible, concerning the factors influencing the stability and activity of T4 phage lysozyme, and, by inference, of proteins in general. We also propose to compare, insofar as possible, the structure and activity of T4 phage lysozyme with that of hen egg-white lysozyme. The structure of Embden Goose lysozyme, which is not homologous with phage lysozyme or hen egg-white lysozyme, will be determined and compared with other known lysozyme structures in order to determine the evolutionary relation between these molecules.